3/3/2013 4:30 PM PST

Bode Miller's got some lady trouble -- to say the least -- his new baby mama just filed a police report against his new wife, Morgan Beck, for aggravated harassment ... TMZ has learned.

Sara McKenna -- who popped out Bode's kid last month -- filed the report Thursday with the NYPD, claiming Morgan (a pro volleyball champ) has been attacking her online with threatening messages.

Sources close to the situation tell TMZ, Sara told cops that Morgan sent out a slew of negative tweets about her under fake accounts -- calling her names like "c**t" and "bitch" -- then accused Sara of kidnapping Bode's kid and threatened to have her custody rights taken away.